Transgenic plants and transformed plant cells have been used for producing recombinant proteins, such as enzymes, antibodies, vaccines, and therapeutic proteins. See, e.g., Fischer et al., 2000, Transgenic Res. 9: 279-299; Giddings et al., 2000, Nat. Biotechnol. 18: 1151-1155; Stoger et al., 2002, Curr. Opin. Biotechnol. 13: 161-166; and Ma et al., 2003, Nat. Rev. Genet. 4: 794-805. The plant-based expression systems offer several advantages over others. For example, they (1) have very little risk of contamination with mammalian pathogens or bacterial toxins, (2) are capable of protein post-translation modification, (3) are more economic than bioreactor-based systems, (4) can be scaled up at relatively low costs, (5) can be developed within a short timeframe, and (6) require no or partial purification if the recombinant proteins are used directly as foods, feed supplements, or raw materials for use. In particular, since plant cells are able to properly process recombinant mammalian proteins, these systems have been used to produce functional and complex mammalian proteins (Sijmons et al., 1990, Biotechnology 8: 217-221; Hiatt et al., Nature 342: 76-78 and During et al., Plant Mol. Biol. 15: 281-293). Nonetheless, they also have limitations. In many cases, the amount of expressed protein ranges from 0.001% to 0.1% of total soluble proteins (Daniell et al., 2001, Trends Plant Sci. 6: 219-226), which is too low for commercial production. Thus, there is a need for a high yield plant expression system.